In the field of extrusion molding of rubber or plastic compounds with use of screw-type cold feed extrusion molding machines, difficulties have thus far been encountered in precisely controlling the sizes of molded products. In order to obviate such difficulties, an attempt has been made to provide in an extrusion molding machine such control means as to regulate the revolution speed of the feed screw incorporated in the machine and to thereby control the sizes of the products issuing from the die. The size control means includes a pressure sensor located at the die of the extrusion molding machine to detect the pressure of the material pressurized by the feed screw being driven for rotation in the extrusion molding machine. The pressure sensor produces an electric signal indicative of the pressure of the material thus detected and the control means is operative to control the revolution speed of the feed screw on the basis of such a signal. A drawback has however been pointed out in connection with control means of this nature in that the control means must be constructed and designed individually depending upon the types, shapes and sizes of the products to be extruded since the dies per se must be exchanged with other ones for producing articles of different types, shapes and sizes. Another drawback of the prior-art size control means is that the pressure sensor located at the die of the extrusion molding machine is not precisely responsive to the pressure applied to the material at the outlet end of the die and, for this reason, the control over the sizes of the extruded products on the basis of the signals produced by the sensor is not fully reliable.
Whereas, it is well known in the art that the sizes of extrusion molded articles produced by a screw-type cold feed extrusion molding machine can not be controlled precisely even when the material to be extruded is supplied to the extrusion molding machine at a precisely controlled rate. Various research and development efforts have been made by the inventors of the present invention to clear up the causes of this. The efforts have made it clear that the contradiction as above noted is accounted for by the fact that the material supplied to a cold feed extrusion molding machine tends to stagnant and form a lump or "bank" in the neighborhood of the inlet of the extrusion molding machine and thus partially fails to be fed to the feed screw. If the bank thus formed in the inlet of the extrusion molding machine grows larger, the molding material subsequently supplied to the extrusion molding machine is impeded from smoothly entering the machine and might fail to reach the die, occasionally giving a rise to a decrease in the delivery rate of the extrusion molding machine. The inventors of the present invention have further ascertained that the presence of a bank in the inlet of a cold feed extrusion molding machine causes a fluctuation in the load current of the motor driving the feed screw of the extrusion molding machine if the feed screw is driven for rotation at a constant speed. This means that the presence of a bank in a cold feed extrusion molding machine can be detected through detection of a change in the load current of the motor in the extrusion molding machine.
It is, accordingly, an important object of the present invention to provide an improved method of precisely controlling the screw-drive and conveyor-drive motors of a screw-type cold feed extrusion molding machine which is subject to formation of a bank of the molding material stagnant in the inlet of the extrusion unit.
It is another important object of the present invention to provide a method of controlling the screw-drive and conveyor-drive motors of a screw-type cold feed extrusion molding machine through detection of the load current of the motor for driving the feed screw and the take-off conveyor of the extrusion molding machine at controlled speeds in the presence of a bank of the molding material in the inlet of the extrusion unit.
It is still another important object of the present invention to provide in a screw-type cold feed extrusion molding apparatus an improved control device for controlling the screw-drive and conveyor-drive motors of the extrusion molding apparatus through detection of the formation of a bank of the molding material stagnant in the inlet of the extrusion unit.
It is, yet, still another important object of the present invention to provide in a screw-type cold feed extrusion molding apparatus a control device for controlling the screw-drive and conveyor-drive motors of the extrusion molding apparatus through detection of the load current of the motor.